Let me point out, as a citizen and resident of NL, that every single individual is PERSONALLY responsible for your own tax payments. No excuses, no exeptions, nothing.
Of course you can have others (such as tax experts) fill it in for you, but still, your own bloody responsibility.
If you discover, as an employee, that your employer is evading tax payments: point it out.
Of course, in this particular case, this would have meant that RYR would terminate your contract, and this dutch RYR pilot would have been without a job. So be it.
The Netherlands has a very efficient tax system. There is NO CHANCE that anyone, or any company such as Ryanair, could ever expect to be able to beat the tax system.
This isn't Greece, or Albania.
The only reason why Ryanair was not held accountable by the judge in this case, is because Ryanair was officially not a party in these civil proceedings.
So: SUE RYR. It is the only option. Hold them accountable.